The CATOFIN® process converts aliphatic hydrocarbons to their corresponding olefins over a fixed-bed chromia alumina catalyst. For example, it can be used to produce isobutylene, propylene or amylenes from isobutane, propane or isopentanes, respectively. The process is an adiabatic, cyclic process. Each cycle comprises several steps, including catalyst reduction, dehydrogenation, purging of the remaining hydrocarbon from the reactor, and finally a regeneration step with air. The cycle then starts again with the reduction step.
The dehydrogenation reaction is highly endothermic. Therefore, the temperature of the catalyst bed decreases during the dehydrogenation step. This decrease in temperature causes a decrease in paraffin conversion. In order to reheat the catalyst bed and remove coke that has deposited on the catalyst during the dehydrogenation step, the reactor is purged of hydrocarbon and then undergoes a regeneration step with air. Heat is provided to the bed by the hot air that passes through the bed and also by the combustion of the coke deposits on the catalyst. Reduction of the catalyst, with a reducing gas such as hydrogen, prior to dehydrogenation step also provides some additional heat. As flow in the reactor is usually from top to bottom and coke deposits to a larger amount at the reactor inlet, there is a tendency for the top of the bed to be hotter than the bottom of the bed. Also, the coke distribution in the catalyst bed, which is not easily controlled, affects the amount of heat added at each location and the resulting catalyst bed temperature profile. These factors make control of the temperature profile in the bed difficult. Hydrothermal stability of catalysts used in the CATOFIN® process is usually the limiting factor for their lifetime use, and thermal stability as well as high selectivity to the olefin are desired qualities.
Aluminum oxide, or alumina, is a commonly used catalyst carrier. The properties it displays vary, depending on its preparation, purity and thermal history. There is a variety of types of alumina, with varying surface areas, pore size distributions, surface acidic properties and crystal structures. Examples include gibbsite (along with its three structural polymorphs bayerite, doyleite and nordstrandite), boehmite and diaspore. Boehmite alumina crystals dehydrate and form a variety of polymorphs depending on the temperature of heating. Alumina produced by dehydration of boehmite exists as γ-alumina between approximately 500 and 850° C., δ-alumina between 850 and 1050° C., θ-alumina between 1050 and 1150° C. and α-alumina above 1150° C. Bayerite is a trihydrate form of alumina with dehydrates to η-alumina between approximately 300 and 500° C., θ-alumina between 850 and 1150° C. and α-alumina above 1150° C.
There are also various stabilizers that can be used with alumina. This includes alkaline earth metals and rare earth metals, as well as other elements such as zirconium. For example, the use of alkaline earth metal oxides is discussed in US Patent Publication No. US 2010/0312035.